Basic Linux Shell Scripting For DevOps

What is Shell Scripting?

Linux shell scripting allows you to automate tasks, manage systems, and streamline development processes. Shell scripts are essentially sequences of commands stored in a file, which can be executed like a program.

Creating and Running a Shell Script:

You need a text editor like: vim, nano, etc. To create shell script.

1. ubuntu@ip-172-31-23-251:~$ vim hello.sh: this command will open a file of name hello.sh where in which we can write our shell script.

2. Write the following in a file named hello.sh:

#!/bin/bash

echo "Hello! Devops"

~
~
-- INSERT -- 5,1 All

Shell scripts typically start with a “shebang” (#!) that tells the system which interpreter to use for the script. press 'i' to insert and write into the file

3. Press 'esc' and then write column ':wq' . To Save the file and exit the editor.

   ubuntu@ip-172-31-23-251:~$ ls

   ubuntu@ip-172-31-23-251:~$ vim hello.sh

   ubuntu@ip-172-31-23-251:~$ ls

   hello.sh

4. check the permissions of hello.sh by following command

   ubuntu@ip-172-31-23-251:~$ ls -l

   total 4

   -rw-rw-r-- 1 ubuntu ubuntu 34 Sep 3 07:09 hello.sh

5. hello.sh don't have executable permission give it executable permission by following command

   ubuntu@ip-172-31-23-251:~$ chmod +x hello.sh

   ubuntu@ip-172-31-23-251:~$ ls -l

   total 4 -rwxrwxr-x 1 ubuntu ubuntu 34 Sep 3 07:09 hello.sh

   ubuntu@ip-172-31-23-251:~$ ls

   hello.sh

6. hello.sh have given the executable command. Run the script file by following command

   ubuntu@ip-172-31-23-251:~$ ./hello.sh

   Hello! DevOps

   ubuntu@ip-172-31-23-251:~$

Lets Move on to Basics:

Variables:

#!/bin/bash

#'name' is a variable

name="DevOps Engineer"

echo "Hello, $name!"

Input from User:

#!/bin/bash

echo "enter the name:"

read username

echo "You entered $username"

Example: Write a script to create a new user

#!/bin/bash

read -p "Enter username: " username

echo "You entered $username"

sudo useradd -m $username

echo "New user added"

after running the script you can check if user is added or not by this command:

$ cat /etc/passwd

Arguments:

./variables.sh huzefa viewer

variables.sh is argument number 1 donated as $0 in script

huzefa is argument number 2 donated as $1 in script

viewer is argument number 3 donated as $2 in script

Let me show you how these arguments can be used in a shell script:

#!/bin/bash

echo "Print argument number 1: $0"

echo "Print argument number 2: $1"

echo "Print argument number 3: $2"

Conditional Statements (if-else):

Conditional statements are used to make decisions in your script.

#!/bin/bash

if [ -f "/etc/passwd" ]; then

echo "/etc/passwd exists"

else

echo "/etc/passwd does not exist"

fi

Loops (for and while):

For Loop: The for loop is used to iterate over a list of items

Following script will make 5 folder using for loop:

#!/bin/bash

for (( num=1; num<=5; num++ ))

do

mkdir "demo$num"

done

Another example;

#!/bin/bash

for (( num=$2; num<=$3; num++ ))

do

mkdir "$1$num"

done

Following command will run the above script and you can create multiple folders of a specified range

$ ./for_loop.sh day 1 90

While Loop:

The while loop runs as long as the condition is true:

#!/bin/bash

counter=1

while [ $counter -le 5 ]

do

echo "Counter: $counter"

((counter++))

done

Functions in Shell Scripts:

Functions allow you to break down your script into smaller, reusable pieces.

#!/bin/bash

greet() {

echo "Hello, $1!"

}

greet "DevOps Engineer"

Scheduling Scripts with Cron:

To automate script execution, you can use cron jobs. For example, to run a script every day at 5 AM, edit the crontab:

crontab -e

Add the following line:

0 5 * * * /path/to/your/script.sh

Best Practices for Shell Scripting in DevOps:

1. Use meaningful names for variables and functions.

2. Use comments to explain complex sections of your script.

3. Error handling: Check for errors and handle them gracefully using exit statuses ($?).

4. Log actions: Include logging in your scripts for easier debugging and auditing.

5. Test scripts thoroughly before deploying them in production environments.